Refactor CLI initialization (#562)

This cleans up the CLI code significantly.
Also makes some of this re-usable for providers who want to do so.

This also removes the main.go from the top of the tree of the repro,
instead moving it into cmd/virtual-kubelet.
This allows us to better utilize the package namespace (and e.g. mv the
`vkubelet` package to the top of the tree).

Gopkg.toml

@@ -33,10 +33,6 @@
   name = "github.com/spf13/cobra"
   version = "0.0.1"
 
-[[constraint]]
-  name = "github.com/spf13/viper"
-  version = "1.0.0"
-
 [[constraint]]
   name = "github.com/google/uuid"
   version = "0.2.0"